1. Field of the Invention
The present invention relates to a source driver for providing signals to source lines of a liquid crystal panel, and to a method for leveling out variations in source driver outputs.
2. Description of Related Art
FIG. 10 illustrates a configuration of a source driver 72 and an example of its connection with a liquid crystal panel 70. Liquid crystal cells are arranged in a grid pattern in the liquid crystal panel 70. Each liquid crystal cell includes a liquid crystal and a transistor for applying to the liquid crystal cell a voltage corresponding to colors to be displayed.
As shown in FIG. 10, the gate terminals of the transistors of liquid crystal cells arranged in a horizontal line are connected to a gate line 78 arranged in a horizontal direction; and the source terminals of the transistors of liquid crystal cells arranged in a vertical line are connected to a source line 74 arranged in a vertical direction. The gate lines 78 are connected to a gate driver 76 which provides signals sequentially to the gate lines 78. The source lines 74 are connected to a source driver 72 which provides signals to the source lines 74. When a signal is provided from the gate driver 76 to a gate line 78, the transistors (liquid crystal cells) connected to the gate line to which signal is provided are selected. When signals are provided from the source driver 72 to the source lines 74 in this state, the selected transistors (liquid crystal cells) are driven at voltages corresponding to the provided signals.
A configuration of the source driver 72 will be briefly described hereafter. The source driver 72 comprises a shift register 86, buffers A 84, buffers B 85, D/A converters (digital-to-analog converters) 82, and output amplifiers 80. Each source line 74 is provided with a buffer A 84, a buffer B 85, a D/A converter 82, and an output amplifier 80. The buffers A 84 each store gray scale data of the pixels of one horizontal line constituting an image to be displayed on the liquid crystal panel 70. The gray scale data are sequentially stored in the buffers A 84 in accordance with a CLOCK signal with the use of the shift register 86. The gray scale data stored in the buffers A 84 is sent to the buffers B 85 with the use of a LOAD signal. The D/A converter 82 converts the gray scale data (digital signals) of the buffers B 85 into analog signals. The analog signals of the D/A converters 82 are amplified by the output amplifiers 80 to be output to the source lines 74.
FIG. 11 shows an example of a portion including adjacent four output amplifiers in the source driver 72 for the liquid crystal panel shown in FIG. 10. Analog outputs from the D/A converters 82 are input into the four output amplifiers AmpA, AmpB, AmpC, and AmpD through wirings DALA, DALB, DALC, and DALD, respectively. The outputs from the D/A converters 82 are amplified respectively by the four output amplifiers AmpA, AmpB, AmpC, and AmpD, respectively, and output to the source lines SLA, SLB, SLC, and SLD, respectively.
If all the gray scale data sent from the buffers B 85 to the D/A converters 82 are at a uniform level, all the analog voltages converted in the converter 82 are also at a uniform level. Therefore, uniform-level voltages are considered to be provided to all the source lines 74. However, voltage levels may vary depending on offset voltage properties, gain properties, and the like in each output amplifier 80. For this reason, even if the gray scale data of all the pixels are at a uniform level, different voltages are provided to source lines 74. Such voltage level differences causes gray level differences, which may often appear to us to be vertical-streak noise on the liquid crystal panel.
In order to eliminate vertical streak noise, a method of sorting out a source driver having small variations in the characteristics of the output amplifiers may be employed. If the variation in the characteristics of the output amplifiers is small and all the gray scale data are at a uniform level, substantially uniform voltages can be provided to all the source lines 74.
However, the sorting of a source driver having small variations in the characteristics of the output amplifiers leads to the reduction of yields and the increase in costs.
An object of the present invention is to level out variations in output voltage levels caused by the difference in the characteristics of the output amplifiers of a source driver, and to reduce vertical streak noise on the display screen.
A source driver for a liquid crystal panel according to the present invention comprises output amplifier switching means for switching an output amplifier, which amplifies analog signal and provides it to source lines of a liquid crystal panel, to another output amplifier at regular time intervals. Such a source driver can level out the output variations caused by the difference between individual output amplifiers, by switching among the output amplifiers, which amplify the signal and provide it to the source lines, at regular time intervals.
A method for leveling out output variations in a source driver for a liquid crystal panel according to the present invention comprises an output amplifier switching step for switching an output amplifier, which amplifies analog signal and provides it to source lines of a liquid crystal panel, to another output amplifier at regular time intervals.